Dilute acid hydrolysis to remove hemicellulose from lignocellulosic materials is one of the most developed pretreatment techniques for lignocellulose and is currently favored (Hamelinck et al., 2005) because it results in fairly high yields of xylose (75% to 90%). Conditions that are typically used range from 0.5% to 1.5% sulfuric acid and temperatures above 160° C. The high temperatures used result in significant levels of thermal decomposition products that inhibit subsequent microbial fermentations (Lavarack et al., 2002). High temperature hydrolysis requires pressurized systems, steam generation, and corrosion resistant materials in reactor construction due to the more corrosive nature of acid at elevated temperatures.
Low temperature acid hydrolyses are of interest because they have the potential to overcome several of the above shortcomings (Tsao et al., 1987). It has been demonstrated that 90% of hemicellulose can be solubilized as oligomers in a few hours of acid treatment in the temperature range of 80° C. to 100° C. It has also been demonstrated that the sugars produced in low temperature acid hydrolysis are stable under those same conditions for at least 24 hours with no detectable degradation to furfural decomposition products. Finally, sulfuric acid typically used in pretreatments is not as corrosive at lower temperatures. The use of lower temperature acid pretreatments requires much longer reaction times to achieve acceptable levels of hydrolysis. Although 90% hemicellulose solubilization has been shown (Tsao, 1987), the bulk of the sugars are in the form of oligomers and are not in the monomeric form. The organisms currently favored in subsequent fermentation steps cannot utilize sugar oligomers (Garrote et al., 2001) and the oligomer-containing hydrolysates require further processing to monomers, usually as a second lower severity acid hydrolysis step (Garrote et al., 2001).
Other acidic pretreatment methods include autohydrolysis and hot water washing. In autohydrolysis, biomass is treated with steam at high temperatures (˜200° C.), which cleaves acetyl side chains associated with hemicellulose to produce acetic acid that functions as the acid catalyst in an acid hydrolysis. Because acetic acid is a much weaker acid than sulfuric acid, below 240° C. the hemicellulose is not completely hydrolyzed to sugar monomers and has high levels of oligomers (Garrote et al., 2001). In hot water washing, biomass is contacted with water (under pressure) at elevated temperatures 160° C. to 230° C. This process can effectively hydrolyze greater than 90% of the hemicellulose present and the solubilized hemicellulose is typically over 95% in the form of oligomers (Liu and Wyman, 2003). Following these pretreatments, it is often necessary to effect further depolymerization of the oligomeric hemicelluloses to monomer sugars, which can be accomplished using a variety of catalysts including, liquids, solids, vaporous acids and alkalis, and enzymes.